1. Field of the Invention
The present invention relates to a photomultiplier of the so-called head-on or end-on type, into which light to be detected is incident through a closed end of a tubular closed container.
2. Related Background Art
FIG. 1 shows a head-on type photomultiplier which is often conventionally used. This photomultiplier has a light entrance window 2 which is a closed end of a glass bulb 1 of a transparent tubular closed container and through which light L to be detected enters the container. The incident light L passing through the light entrance window 2 reaches a photocathode 3 formed inside the light entrance window 2, whereby photoelectrons are emitted. A focusing electrode 4 guides the emitted photoelectrons to an electron multiplying unit 5. The electron multiplying unit 5 as shown is comprised of a plurality of box-and-grid type dynodes 6 combined in multiple stages. The electron multiplying unit 5 successively multiplies the photoelectrons by the secondary emission effect, and the thus multiplied electrons are collected as output signal by an anode 7.
In the conventional photomultiplier as described, aluminum is vapor-deposited over an internal surface of a side wall 8 of the glass bulb 1 in order to maintain the potential of the photocathode 3. The aluminum vapor-deposited film 9 is connected with the photocathode 3 at a part thereof. The aluminum vapor-deposited film 9 is formed over the entire internal surface of side wall 8 within the region between the light entrance window 2 and the focusing electrode 4 to prevent light from the side from entering the glass bulb 1.
The existence of aluminum vapor-deposited film 9, however, causes a decrease in sensitivity of photomultiplier, because in addition to the incident light which passes into an effective area in the light entrance window 2, incident light L' is directed obliquely and outwardly at the periphery of effective area and therefore cannot be detected. As clearly shown in FIG. 2, a part of incident light L' passes obliquely at the periphery of light entrance window 2 and exists from the side wall 8 of glass bulb 1. The remainder is reflected by the external surface of side wall 8 towards the inside of the glass bulb 1, but is stopped from entering the inside by the aluminum vapor-deposited film 9.
A countermeasure against this problem may be use of a photomultiplier having a side photocathode 10 which is formed by extending the photocathode 3 to above the internal surface of side wall 8 of glass bulb 1, as shown in FIG. 3. Using the photomultiplier of such type, a part of light L' could go into the side photocathode 10 and be detected as electrons. It is, however, impossible that all light L' impinge on the side photocathode 10, and therefore a part thereof is inevitably lost out of the side wall 8. In addition, the photomultiplier of FIG. 3 also detects light L" going toward the side of glass bulb 1, which hinders detection of only light coming into the front of light entrance window 2.